The present invention relates to a pneumatic tire having a layer of a starch/plasticizer composite for prevention, or retardation, of air permeation from its inner chamber into the remainder of the tire.
A pneumatic rubber tire is conventionally of a toroidal shape and comprised of a carcass with a cavity, or chamber, for containing air in which its closure is typically completed with a rigid rim. Such cavity might sometimes be referred to as the tire""s xe2x80x9cinner air chamberxe2x80x9d. Such pneumatic tire/rim assembly is well known.
The rubber composition which forms the inner surface of a pneumatic tire, as the exposed surface of the aforesaid cavity, sometimes referred to as an xe2x80x9cinnerlinerxe2x80x9d, is typically composed of an elastomeric composition designed to prevent, or retard, the permeation of air and moisture into the tire carcass from the tire""s inner air chamber.
Butyl rubber is typically relatively impermeable to air and moisture and is often used as a major portion of the innerliner composition and can be a form of butyl rubber or halobutyl rubber such as, for example, bromobutyl rubber. For example, see U.S. Pat. No. 3,808,177. Butyl rubber, while containing a minor amount of units derived from a diene such as, for example isoprene, is not considered herein as being a diene-based rubber since it contains less than 15 percent of its content derived form a diene monomer and, therefore, sulfur vulcanizes at a much slower rate than diene-based elastomers which contain at least, for example, 30 percent of their elastomer content derived from diene monomers. Such innerliner concept is well known to those skilled in such art.
However, the carcass rubber is usually composed of one or more elastomers which are composed of a substantial portion comprised of diene-based elastomers.
Diene-based elastomers would also be desirable for the tire innerliner composition to enhance sulfur vulcanization compatibility between the innerliner rubber and carcass rubber which itself is normally composed of a diene hydrocarbon-based rubber composition.
Also, such butyl innerliners add weight to a pneumatic tire because they must have a sufficient thickness to effect satisfactory reduction in air permeability. While such thickness (e.g.: usually at least about three millimeters) results in a relatively small addition of weight and thickness to the tire, such weight and thickness add, while also small, a nevertheless significant, hysteresis loss to the tire carcass resulting in an increase in rolling resistance of the tire and, accordingly, an increase on fuel consumption for a vehicle on which tires are utilized.
Numerous suggestions have been made for using various materials in place of, or commensurate, with butyl innerliners for pneumatic tires.
For example, polyvinylidene chloride films have been suggested for tire innerliner applications for reduced permeation.
Also, thermoplastic polyester elastomers comprised of a block copolymer of polybutylene terephthalate and polyoxyalkylene diimide diacid resin film and blends of such containing a blend of rubber particles therein has been suggested. See U.S. Pat. No. 5,738,158.
Such starch composite presents a relatively low air permeability, and it is desired to create a successful adaptation of such a composite for use in a tire innerliner.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d where used herein, and according to conventional practice, refers to xe2x80x9cparts of a respective material per 100 parts by weight of rubber, or elastomerxe2x80x9d.
In the description of this invention, the terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d where used herein, may be used interchangeably, unless otherwise prescribed. The terms xe2x80x9crubber compositionxe2x80x9d, xe2x80x9ccompounded rubberxe2x80x9d and xe2x80x9crubber compoundxe2x80x9d, where used herein, are used interchangeably to refer to xe2x80x9crubber which has been blended or mixed with various ingredients and materialsxe2x80x9d and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
The term xe2x80x9ccarbon blackxe2x80x9d as used herein means xe2x80x9ccarbon blacks having properties typically used in the reinforcement of elastomers, particularly sulfurcurable elastomersxe2x80x9d.
A reference to a range of values is intended to be inclusive of such values. For example, a range recited as being from one to three would be intended to include the values of one and three.
A reference to an elastomer""s Tg refers to its glass transition temperature, which can conveniently be determined by a differential scanning calorimeter at a heating rate of 10xc2x0 C. temperature increase per minute.